This invention relates to a method and apparatus for determining both the pore-size characteristics and integrity of porous membranes and membrane filters and employs miscible liquids, at least one of which is electrically conductive, which do not spontaneously wet the porous structure of the membrane. Specifically, this invention relates to a method and apparatus for determining the pore-size characteristics and/or integrity of a membrane or filter based upon the use of electrically conductive liquids and electrical measurements.
Presently, the pore-size characterization and determination of integrity for membranes and filters, in general, are performed via procedures which are referred to as, among other things, "air-flow porosimetry", the "bubble-point test" or "bubble-point determination", and the "diffusion test". In addition, hydrophobic membranes, specifically, can also be characterized and tested by procedures referred to as, among other things, the "water intrusion-pressure determination" and the "water-flow test" or "water-intrusion test".
The bubble-point test and air-flow porosimetry utilize a liquid which spontaneously wets the membrane in question, and are based on the fact that subsequent attempts to displace the wetting liquid with a gas require that the gas pressure be elevated to some critical level dependent on the size of the pores, or the size of defects if present, in order to overcome the surface-tension forces holding the liquid in the pores. The equation for this critical pressure, defined as the bubble-point pressure, is a variation of the Young-Laplace equation for capillary pressure drop, in this application often called the Washburn equation: EQU P.sub.BUBBLE POINT =4K.sigma. cos(.theta.)/d (Eqn. 1)
where
P.sub.BUBBLE POINT =bubble-point pressure PA1 K=the pore perimeter (shape) correction factor PA1 .sigma.=surface tension of the liquid PA1 .theta.=contact angle of the liquid against the solid PA1 d=the diameter of the pore PA1 1. is affected by the membrane or filter only via the pore size, PA1 2. results in a smaller range of measured values at imposed pressures less than the intrusion pressure, which in turn PA1 3. makes changes in the measured value more obvious as the imposed pressure is taken through the range over which the various sizes of pores are intruded, resulting in a more precise description of the relative pore-size distribution, and PA1 4. makes at a single pressure below the required intrusion pressure, the measured value for a defective filter more obviously different from those for normal integral filters.
Equation 1 is rarely actually used to quantitatively calculate a pore size from empirical bubble-point data, since the pore perimeter correction factor, K, is rarely known independently. Instead, since the equation indicates that the bubble point is inversely related to the pore diameter, it is used as a justification to qualitatively rank the relative pore size of membranes according to their bubble-point pressures. Further, since particle retention efficiency is related to the pore size, Equation 1 also serves as a conceptual justification for empirically correlating the retention efficiency of membranes of various pore sizes to their bubble points. Membrane manufacturers have taken advantage of this retention-vs.-bubble-point relationship to identify the critical bubble point required for a desired level of retention, and filter users conduct bubble-point determinations to confirm that the filter in question is integral and of the appropriate pore size.
Air-flow porosimetry and a visual version of the bubble-point test for membrane samples are described by ASTM Method F316-86. In general, the bubble-point test is performed by prewetting the membrane with the liquid to be used and mounting the membrane in a specially designed holder which allows a visually observable layer of liquid to be placed on the downstream, i.e. upper side of the membrane. In the case of a bubble-point test of an enclosed filter, the filter is flushed with the liquid to wet the membrane. The pressure of air or other gas on the upstream side of the membrane is then increased, and the downstream liquid layer or the outlet from the enclosed filter is observed for the formation of continuous streams of bubbles. The pressure at which these first appear is called the bubble-point pressure of the sample.
For relatively large filters, which experience significant diffusion rates at pressures below the bubble point as discussed below, a more analytical method is used to determine the bubble-point pressure. In this case, the rate of flow of gas through the filter is measured as a function of the imposed gas pressure, and the pressure at which the flow makes a transition from relatively low flow rates, which is indicative of diffusion only to significantly higher flow rates, which is indicative of bulk gas flow through pores or defects is referred to as the bubble-point pressure of the filter.
Porosimetry is used to determine the relative pore-size distribution of a membrane or membrane filter. In this procedure, the flow rate of gas through a pre-wetted membrane at a particular gas pressure is divided by the flow rate of gas through an initially dry identical membrane at the same pressure. The resulting mathematical ratio, R, is plotted as a function of imposed pressure, and the first derivative of this function, dR/dP, yields a bubble-point pressure distribution, which, via the bubble-point equation shown above, also indicates the relative distribution of pore sizes, as well.
The diffusion test is used primarily for relatively large filters and indicates whether or not the filter is integral via a measurement of the gas flow rate through the filter when exposed to a constant upstream gas pressure equal to, or slightly below, the minimum bubble-point pressure required for the filter. Similar to a bubble-point test, the filter is pre-wet with the intended liquid. At a properly selected test pressure, the measured flow rate will be relatively low when the filter is integral and of the appropriate pore size. The source of gas flow through an integral filter at pressures below the actual bubble point of the filter is dissolution of gas into, diffusion through, and re-evaporation from the liquid filling the pores, without forcing the liquid out of the pores. In such a test, a filter with an undesirably large pore size or with a defect will exhibit relatively large gas flow rates as a result of the test pressure being in excess of the filter's actual bubble point.
The water intrusion-pressure determination for hydrophobic filters is conducted via a method similar to the bubble-point determination with the exception that in an intrusion-pressure determination, the membrane is initially dry and the pressure at which water intrudes into and through the membrane is noted. The pressure at which this occurs is, like the bubble-point (Eqn. 1), inversely related to the pore size and is, therefore, justifiably used to indicate the relative pore size of various membranes and can be correlated to retention efficiency: EQU P.sub.INTRUSION =-4K.sigma. cos(.theta.)/d (Eqn. 2)
The negative sign results from the fact that the contact angle of water on a hydrophobic solid is greater than 90 and thus the cosine is negative.
The water-flow test, like the diffusion test, is conducted at a constant pressure. In this case, the upstream side of a dry hydrophobic filter is exposed to water at a constant pressure equal to, or slightly below, the minimum intrusion pressure required for the filter, and a measurement of the water flow rate into the filter's housing is made. This measurement is conveniently performed on the upstream side of the filter, either by measuring the flow of water directly with a flow meter, or by measuring the pressure as a function of time in an adjoining gas space and calculating the gas expansion rate, which just equals the water flow rate. The latter is presently performed by automated testing devices. However, it is also possible to measure the downstream gas flow rate and equate this to the upstream water flow rate since the upstream water, membrane, and downstream air all move approximately together in a piston-like fashion at pressures below the intrusion pressure of the membrane.
Unlike the diffusion test, the relatively low water flow rates observed in a water-flow test conducted at a pressure below the normal water intrusion pressure are not due to gas diffusion. Instead, the water flow results from water flowing forward to fill the volume vacated by the shifting, compaction, and stretching of a pleated membrane structure common to many large-area filters. In an actual water-flow test, an observed low flow rate is indicative of pleat compaction only, and thus of an integral filter, and a large flow rate is indicative of water flowing through undesirably large pores or a defect which is successfully intruded at the test pressure.
The water intrusion-pressure determination and the water-flow tests offer an important advantage for hydrophobic filters as compared to a bubble-point determination and diffusion test: the latter two require the use of a low-surface-tension liquid, e.g. an alcohol or an alcohol-water mixture, to initially wet the hydrophobic filter, and the use of such liquids pose safety and disposal problems that do not exist for the water-flow test nor the water intrusion-pressure determination.
In spite of the above stated advantage of the water intrusion-pressure determination and the water-flow test, an additional feature is desirable for these tests. As mentioned above in the case of a pleated filter construction, the water flow into the housing experienced by a filter exposed to a pressure below the water intrusion pressure is due to the compaction of the pleated-membrane structure, which in no way is related to the pore size or the pore-size distribution of the membrane. Therefore, it is desirable to use, instead of a water flow-rate measurement, a different measurement which